The present invention is directed to a method for increasing production of methane from a coal seam and, more particularly, to inputting air and produced methane into a fuel cell power system and injecting the fuel cell power system exhaust comprising oxygen-depleted air into the coal seam.
Methane is the primary component of natural gas, a widely used fuel source. Subterranean coal seams contain substantial quantities of methane adsorbed onto the coal surface. Methane is produced from the coal seams for use as a fuel.
A wellbore is drilled into a subterranean coal seam for producing methane. The pressure difference between the coal seam and the wellbore provides the driving force for flowing methane to the wellbore. Reduction of pressure in the coal seam as methane is produced increases desorption of methane from the coal surface, but, at the same time, deprives the system of the driving force necessary to flow methane to the wellbore. Consequently, this method looses its effectiveness over time for producing recoverable methane reserves.
A method for producing coal seam methane by injecting an inert gas into the coal seam is disclosed in U.S. Pat. No. 4,883,122 (Puri et al.). The inert gas injection reduces methane partial pressure for desorbing methane from the coal surface. The inert gas can maintain or increase total pressure in the coal seam. This method is effective for increasing the total amount and rate of methane production from the coal seam.
The Puri et al. patent discloses that nitrogen is the preferred inert gas for the method for increasing methane production. Nitrogen must be transported to the methane production site or an air separation plant must be provided on-site to separate nitrogen from air.
The Puri et al. patent further discloses that air is a suitable source of nitrogen for the method for increasing methane production. However, injection of oxygen, which is a component of air, into a coal seam for increasing production of methane can present problems. Oxygen can cause corrosion and rust formation in well casings and other fluid conduits. Also, produced fluids comprising oxygen and methane are potentially flammable and/or explosive. It is desirable to provide an economically attractive method to minimize these potential problems by depleting the oxygen content of air before injecting air into a coal seam for increasing methane production.
An object of the present invention is to provide an economical method for increasing production of methane from a coal seam using oxygen-depleted air. Another object of the present invention is to provide a method for generating oxygen-depleted air for use in increasing production of methane from a subterranean coal seam. Other objects of the present invention will appear hereinafter.
The objects of the present invention can be attained by a method for increasing production of methane from a subterranean coal seam comprising the steps: producing fluids comprising methane from the subterranean coal seam through at least one production well; inputting air, as a source of oxygen, and a portion of the produced fluids comprising methane, as a source of hydrogen, into at least one fuel cell power system for generating fuel cell power system exhaust comprising oxygen-depleted air; and injecting fuel cell power system exhaust comprising oxygen-depleted air into the subterranean coal seam through at least one injection well.
The present invention has the advantages that nitrogen does not have to be transported to the methane production site nor does an air separation plant have to be provided on-site for separating nitrogen from air; part of the methane produced and oxygen from air are used in the fuel cell power system for generating oxygen-depleted air and electricity for on-site needs; a fuel cell power system operates efficiently with essentially no environmentally harmful emissions (no combustion is required); injecting oxygen-depleted air into the subterranean coal seam reduces potential for rust formation and corrosion in piping and wellbore casing; and injecting oxygen-depleted air into the subterranean coal seam reduces potential for flammable or explosive produced fluids.
For purposes of illustration, the method of this invention is described in relation to a fuel cell power system including a fuel processor, a power section, and a power conditioner. The method of the present invention contemplates the use of any fuel processor that can utilize coal seam methane as a fuel. Internal operation of the fuel cell power system components can differ from what is described below.
A test project utilizing coalbed methane as fuel for a fuel cell power system is disclosed in "Coalbed Methane as a Fuel for Fuel Cell Power Plants," A. F. Ellis, et al., 1986 Fuel Cell Seminar. Tests verify that coalbed methane is a suitable alternative fuel for fuel cell power systems designed for natural gas operation.
Methane is often produced from at least two production wells in a subterranean coal seam. In the method of this invention, at least one of the production wells is converted to an injection well or at least one new injection well is drilled for injecting oxygen-depleted air into the coal seam. A fluid comprising oxygen-depleted air is injected through one or more injection wells into the subterranean coal seam. In one embodiment, production of methane from the coal seam is increased because the injected fluid maintains or increases the total reservoir pressure while the partial pressure of methane is reduced. Reduction of methane partial pressure increases desorption of methane from coal. In a preferred embodiment, the injection pressure is maintained substantially constant and the fluids comprising oxygen-depleted air are injected into the coal seam by continuous injection.